Method of making aluminium rolled product having at least one bright surface

ABSTRACT

The invention concerns a method for cold rolling bright aluminium rolled products and comprises:
     (a) Providing a first aluminium alloy rolled product   (b) Rolling said first product with a rolling mill ( 1 ) having at least one work roll ground to a finish ( 11 ), with a roughness Ra less than 1 μm, and using a rolling lubricant ( 51 ) from a first container ( 2 ) comprising less than 150 mg/L of aluminium debris to obtain a second aluminium rolled product having at least one bright surface,   (c) Transferring at least a portion of said rolling lubricant to a second container ( 3 )   (d) Filtering the lubricant from said second container ( 52 ) by using cross flow filtration ( 4 ) through a membrane ( 41 ), to obtain a cleaned rolling lubricant ( 54 ) comprising less than 10 mg/L and preferably less than 5 mg/L of aluminium debris preferentially having a mean diameter of less than 2 μm,   (e) Returning said cleaned rolling lubricant ( 54 ) to the first container.

FIELD OF THE INVENTION

The present invention relates to methods of making aluminium rolledproducts having at least bright surface, for example for lightingreflector applications.

BACKGROUND OF THE INVENTION

Bright aluminium sheets have been used for many applications requiringspecific aspect or optical properties for example in solar, automotive,cosmetics, decorative or in lighting applications, such as in reflectorassemblies. For these applications, aluminium sheets must have variousphotometric properties to meet requirements of lighting fixturemanufacturers. Among the most highly sought photometric properties arehigh specular reflection (the amount of light reflected by the sheet, asa percentage of incident light), and low diffuse or non-directionalreflection of light. These properties have been achieved in the priorart by a combination of careful alloy selection for the sheet stockmaterial and one of two cold rolling practices. In one practice, thesheet stock is cold rolled to a standard mill finish, followed by eitherchemical/electrochemical finishing or organic coating. In the otherpractice, at least one surface of aluminium alloy sheet is cold rolledwith a specular or bright cold rolled surface using a work roll that hasbeen polished to a smooth finish. This resultant sheet has high specularor mirror like reflectance and very low diffuse reflectance. Forproviding aluminium rolled products with a bright surface, the surfaceof the working roll used in a rolling mill to roll the product must havea topography that is engineered to provide the desired brightness. Alsoof importance is the difficulty in preventing transfer of the aluminiumonto the rolls. Because of the friction and high rolling pressures used,great care must be exercised in lubricating the frictional interfacebetween the aluminium work piece and the product in order to insureminimum build-up of aluminium on the product while at the same timeattaining a bright product. Also possibly related to the lubricant isthe presence of water stains and particles which are aestheticallyunattractive and their elimination or reduction is valuable.

In order to solve these problems, and provide an efficient andeconomical process for making aluminium rolled product having at leastbright surface, an improved process including improved lubricant andmethod to maintain this improved lubricant has been designed.

SUMMARY OF THE INVENTION

An object of the invention is a method for cold rolling bright aluminiumrolled products comprising:

(a) Providing a first aluminium alloy rolled product

(b) Rolling said first product with a rolling mill having at least onework roll ground to a finish, with a roughness Ra less than 1 μm, andusing a rolling lubricant from a first container comprising less than150 mg/L of aluminium debris to obtain a second aluminium rolled producthaving at least one bright surface,

(c) Transferring at least a portion of said rolling lubricant to asecond container

(d) Filtering the lubricant from said second container by using crossflow filtration through a membrane, to obtain a cleaned rollinglubricant comprising less than 10 mg/L and preferably less than 5 mg/Lof aluminium debris preferentially having a mean diameter of less than 2μm,

(e) Returning said cleaned rolling lubricant to the first container.

DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic diagram of the method of the invention.

FIG. 2 shows the evolution of the content of aluminium debris of therolling lubricant 51 from the first container 2 with the method of theinvention. Start of the cross-filtration process is shown with an arrow.

FIG. 3 shows the evolution of the content of aluminium debris of thecleaned lubricant 54.

DESCRIPTION OF THE INVENTION

Unless mentioned otherwise, definition of standard NF EN 12258 apply.

According to the present invention a method for cold rolling brightaluminium rolled products is provided.

Bright aluminium rolled products are aluminium rolled products having atleast one bright surface. By bright surface it is meant a surface havingcontrolled optical properties. Bright surfaces include mirror, rolledbright, semi-specular, mat or structured surfaces. In a preferredembodiment of the invention the specular reflectance of the brightsurface, typically a mirror or rolled bright surface, measured forexample with a glossmeter according to standard ISO7668 at an angle of20°, is at least 58% and preferably at least 78%, and the diffusereflectance, measured for example with a Ulbricht sphere according tostandard DIN 5036-3, remains less than 36% and preferably less than 8%.In another embodiment of the invention, typically a mat or structuredbright surface, the specular reflectance of the bright surface, measuredfor example with a glossmeter according to standard ISO7668 at an angleof 60°, is between 15% and 75%, typically less than 50%, and the diffusereflectance, measured for example with a Ulbricht sphere according tostandard DIN 5036-3, remains more than 60% and preferably more than 70%.

A first aluminium alloy rolled product is provided. The aluminium alloyrolled product typically has a thickness between 0.1 and 2 mm andpreferably between 0.3 and 1.2 mm. Preferably said aluminium rolledproduct comprises an aluminium alloy from the 1XXX, 3XXX or 5XXX series,as defined by The Aluminum Association, known to those skilled in theart. This first aluminium alloy rolled product may have standard millfinish surfaces or may have at least one bright surface. Bright productsare usually obtained by using a rolling schedule including severalrolling passes with decreasing work roll roughness. This first aluminiumalloy rolled product is rolled with a rolling mill having at least onework roll polished to a finish with a roughness Ra, measured accordingto standard DIN EN ISO 4288, less than 1 μm, and preferentially a smoothfinish with Ra less than 0.1 μm and preferably Ra less than 0.05 μm andeven more preferably Ra less than 0.03 μm. It is noted that work rollspolished to a finish with a roughness Ra between 0.1 and 1 μm areparticularly adapted for mat or textured bright surfaces. In anembodiment both work rolls have a roughness Ra less than 1 μm,preferentially less than 0.1 μm, and preferably Ra less than 0.05 μm oreven Ra less than 0.03 μm. The rolling mill may be of the type duo (tworolls), quarto (4 rolls) or sexto (6 rolls). In a preferred embodimentthe rolling mill is of the type duo, because the roughness of the workroll is then not affected by the roughness of other rolls and becausethe aluminium transfer between the roll and the sheet is easier tomanage. A second aluminium rolled product having at least one brightsurface is obtained by rolling. Preferably the roughness Ra of said atleast one bright surface measured according to standard DIN EN ISO 4288and/or by using white light interferometry is less than 0.1 μm,preferably less than 0.05 μm, and even more preferably Ra less than 0.03μm. In an embodiment said bright surface has a roughness Ra between 0.01μm and 0.02 μm. In another embodiment related to mat or texturedsurfaces the roughness Ra of said at least one bright surface measuredaccording to standard DIN EN ISO 4288 and/or by using white lightinterferometry is more than 0.1 μm or even more than 0.2 μm and lessthan 1 μm, preferably less than 0.5 μm, and even more preferably Ra lessthan 0.4 μm. In another embodiment said bright surface has a roughnessRa between 0.3 μm and 0.5 μm. In yet another embodiment said brightsurface has a roughness Ra between 0.8 μm and 1.2 μm.

A rolling lubricant stored in a first container is used to cool down andlubricate the rolling mill. The present inventors observed that thequality of said bright surface is improved and consistently maintainedif the rolling lubricant comprises less than 150 mg/L of aluminiumdebris, preferably less than 100 mg/L and preferentially less than 50mg/L. During rolling, the content of aluminium debris in the rollinglubricant however continuously increases due to the frictional interfacebetween the steel working roll and the rolled product. The method of theinvention enables to maintain the content of aluminium debris less than150 mg/L during several months of operations of the rolling mill withoutcomplete replacement of the rolling lubricant as only punctual additionsto maintain the total quantity of lubricant are needed. Contrary toother expensive solutions wherein the rolling lubricant is replaced assoon as the content of aluminium debris reaches a value above 150 mg/L,in the method of the invention an efficient filtering process of therolling lubricant is provided. According to the invention at least aportion of said rolling lubricant is transferred to a second container.The portion which is transferred is sufficient to maintain a flow oflubricant enabling a control of the quantity of aluminium debris is thefirst container. The lubricant from said second container is filtered byusing cross flow filtration through a membrane, to obtain a cleanedrolling lubricant comprising less than 10 mg/L of aluminium debrispreferentially having a mean diameter of less than 2 μm and morepreferentially having a mean diameter of less than 1 μm. Crossflowfiltration, also known as tangential flow filtration, is a type offiltration wherein the majority of the feed flow travels tangentiallyalong the surface of the filter, rather than across into the filter. Thesolids debris are rejected into this main flux and are recycled in aworking tank. The filtered liquid goes in the perpendicular direction(crossflow) to this main flux. The two different and perpendiculardirections of the dirty oil and the filtrate enable a very limitedclogging of the filter. Crossflow filtration is different from dead-endfiltration in which the feed is passed through a membrane or bed, thesolids being trapped in the filter and the filtrate being released atthe other end. The present inventors observed that crossflow filtrationis particularly suitable to obtain a significant flow of a cleanedrolling lubricant comprising less than 10 mg/L and preferably 5 mg/L ofaluminium debris for the rolling of bright products. Indeed the presentinventors observed that a particular aspect of the bright productsrolling is that the aluminium debris which are produced are particularlysmall, typically less than 2 μm diameter and are not easily filteredwith standard techniques, such as stack filter with filtration aids,such as diatomaceous earths, clay or cellulose.

Eliminating large particles with a diameter of more than 2 μm and evenmore preferably of more than 1 μm or even of more than 0.5 μm, alsocontributes to the product quality as hard large aluminium debris canproduce defect on the aluminium rolled product surfaces. Advantageouslythe cross flow filtration is carried out through at least onecylindrical ceramic membrane, preferably with a porosity less than 0.3μm and preferentially less than 0.2 μm. The ceramic membranes arepreferably made of alpha alumina, carbide silicon, zirconia or titaniumoxide or combinations thereof. Alternatively, metallic membranes made ofstainless steel with a similar porosity can also be used. In yet anotherembodiment the cross flow filtration is carried out with filter membraneassembly having a plurality of filter membranes dynamically moving. Theceramic membranes can also have an hydrophobic behaviour that limits thehumidity adsorption into the support and helps maintaining high flowrate during operation.

The size and characteristics of the membrane are chosen to removealuminium debris without changing the composition of the rolling oil,particularly functional additives should preferably remain in therolling lubricant.

The cleaned rolling lubricant comprising less than 10 mg/L andpreferably less than 5 mg/L of aluminium debris preferentially having amean diameter of less than 2 μm, is then returned to the first containerwith a flow sufficient to maintain less than 150 mg/L of aluminiumdebris in the first container.

Preferably the water content of the rolling lubricant from said firstcontainer is less than 0.1 g/L and the water content of the cleanedrolling lubricant is less than 0.05 g/L. In an embodiment a waterextraction device is connected to said second container. The waterextraction device may be a degassing device or an absorption device suchas for example a cellulose filter. In another embodiment, thetemperature of the rolling lubricant during cross flow filtration isbetween 90° C. and 100° C. to permit evaporation of water. Maintainingthe water content of the rolling lubricant from the first container toless than 0.1 g/L is advantageous to avoid water stains on the secondrolled aluminium product.

The rolling lubricant temperature from the first container is preferablymaintained between 30° C. and 50° C. Advantageously the temperature ofthe rolling lubricant during cross flow filtration is between 50° C. and100° C. If the temperature of the rolling lubricant during cross flowfiltration is between 90° C. and 100° C., a cooling device may be neededto obtain the desired rolling lubricant temperature in the firstcontainer. In another embodiment the temperature of the rollinglubricant during cross flow filtration is between 50° C. and 70° C.

FIG. 1 is a schematic diagram of devices and flows suitable to carry outthe method of the present invention. A rolling mill 1 with at least onework roll 11 ground to a smooth finish is used to roll a first aluminiumrolled product into a second aluminium rolled product, having at leastone bright surface. A rolling lubricant 51 from a first container 2comprising less than 150 mg/L of aluminium debris is used for rolling.The rolling lubricant is continuously filtered in a loop 21, through aregular stack filter 22. At least a portion of the rolling lubricant 51is transferred to a second container 3. The transfer may be donecontinuously or by batch. The lubricant 52 from the second container 3is filtered by using cross flow filtration 4. A pump 31 is used to feeda loop of cross filtration 42 maintained by a pump 43. The pump 43 issufficient to obtain cross filtration through a cross-filtrationmembrane 41. The rolling lubricant in the section 53 tangentiallycrosses the membrane 41 and a cleaned rolling lubricant 54 comprisingless than 10 mg/L of aluminium debris and preferably less than 5 mg/L ofaluminium debris preferentially having a mean diameter of less than 2μm, is obtained. The cleaned rolling lubricant 54 is returned to thefirst container 2.

EXAMPLE

A duo-rolling mill used for cold rolling bright aluminium rolled hasbeen operated with a method of the invention as described in FIG. 1. Thecross-filtration membrane was made of Ti—Zr oxide with an alpha aluminalayer and had a cut-off of 0.1 μm which enabled that the filteredrolling oil had a mean diameter of less than 2 μm. The mill was usedseveral months without any replacement of the rolling oil, only punctualadditions to maintain the total quantity of lubricant.

The content of aluminium debris of the rolling lubricant 51 from thefirst container 2 and of the cleaned lubricant 54 are provided in FIG. 2et 3, respectively.

1. A method for cold rolling bright aluminium rolled productscomprising: (a) Providing a first aluminium alloy rolled product (b)Rolling said first product with a rolling mill having at least one workroll ground to a finish, with a roughness R_(a) less than 1 μm, andusing a rolling lubricant from a first container comprising less than150 mg/L of aluminium debris to obtain a second aluminium rolled producthaving at least one bright surface, (c) Transferring at least a portionof said rolling lubricant to a second container (d) Filtering thelubricant from said second container by using cross flow filtrationthrough a membrane, to obtain a cleaned rolling lubricant comprisingless than 10 mg/L and optionally less than 5 mg/L of aluminium debrisoptionally having a mean diameter of less than 2 μm, (e) Returning saidcleaned rolling lubricant to the first container.
 2. The methodaccording to claim 1 wherein the water content of the rolling lubricantfrom said first container is less than 0.1 g/L and the water content ofsaid cleaned rolling lubricant is less than 0.05 g/L.
 3. The methodaccording to claim 1 wherein the cross flow filtration is carried outthrough at least one cylindrical ceramic membrane
 4. The methodaccording to claim 1 wherein the cross flow filtration is carried outwith filter membrane assembly having a plurality of filter membranesdynamically moving.
 5. The method according to claim 1 wherein therolling lubricant temperature is between 30° C. and 50° C.
 6. The methodaccording to anyone of claim 1 wherein the cross flow filtration iscarried out at a temperature between 50° C. and 100° C.
 7. The methodaccording to anyone of claim 1 wherein the roughness Ra of said at leastone bright surface measured according to standard DIN EN ISO 4288 and/orby using white light interferometry is less than 0.1 μm, optionally lessthan 0.05 μm, and optionally Ra less than 0.03 μm.
 8. The methodaccording to claim 1 wherein said rolling lubricant from said firstcontainer comprises less than 50 mg/L of aluminium debris.
 9. The methodaccording to claim 1 wherein said aluminium rolled product comprises analuminium alloy from the 1XXX, 3XXX or 5XXX series.
 10. The methodaccording to claim 1 wherein said at least one work roll is ground to asmooth finish with a roughness R_(a) less than 0.1 μm.